MicroRNAs (miRs) are small non-coding RNAs that post-transcriptionally repress gene expression and play important roles in a wide range of biological processes, including cell fate determination and organogenesis during embryonic development. Several miRs that function in cardiovascular development have been studied revealing their importance in the delicate regulation of this developmental process. More miRs involved in this process are being studied and the overall results will broaden our understanding of the formation of cardiovascular system and eventually benefit the diagnosis, prevention and therapy of human cardiovascular diseases. Our long-term objective is to identify these miRs and elucidate the underlying mechanism of how these miRs regulate cardiovascular development. Our preliminary studies have demonstrated that miR-451, a highly conserved miR identified from a screen for cardiogenic miRs, is expressed extensively in endothelial and blood cell lineages during embryogenesis and has distinct functions during in vitro embryonic stem cell differentiation by promoting the differentiation of endothelial and blood cells while blocking the differentiation of cardiomyocytes. This function is achieved at least partially through the downregulation of target Acvr2a and upregulation of Wnt signaling. However, the in vivo function of miR-451 in cardiovascular development has not been investigated. Therefore, the hypothesis of this project is that miR-451 functions in angiogenesis, hematopoiesis and cardiogenesis during embryogenesis by regulating the specification of respective cell lineages. To test this hypothesis, three specific aims are proposed: 1. To identify the cardiovascular phenotype of miR-451 knockout mice. 2. To investigate the role of miR-451 in the specification of endothelial, blood and cardiac cell lineages during development. 3. To elucidate molecular mechanism underlying miR-451's function in cardiovascular developmental. The proposed studies should not only gain insights into the mechanisms of miR-451 functions in cardiovascular development, but also provide valuable information on the cell lineage determination regulated by miRs during early development.